1. Field of the Invention
The present invention relates to coil elements, for example, transformers and common mode choke coils.
2. Description of the Related Art
FIG. 4 is a perspective view which shows an example of a common mode choke coil comprising a coil element, and FIG. 5 is an assembly view of the common mode choke coil shown in FIG. 4. The common mode choke coil 1 is disclosed in Japanese Patent Publication No.8-203737, and includes, as shown in FIG. 4, a composite member 7 disposed on a first magnetic substrate 3, a second magnetic substrate 10 disposed on the composite member 7 and an adhesive layer 8 located therebetween, and external electrodes 11 located on the outer surfaces of the first magnetic substrate 3, the composite member 7, the adhesive layer 8, and the second magnetic substrate 10.
As shown in FIG. 5, the composite member 7 includes a plurality of layers deposited by a thin-film fabricating technique such as sputtering, wherein an insulating layer 6a composed of a non-magnetic insulating material, for example, a polyimide resin and an epoxy resin, is deposited on the first magnetic substrate 3, leading electrodes 12a and 12b are disposed on the insulating layer 6a, an insulating layer 6b is disposed on the leading electrodes 12a and 12b, a coil pattern 4 and a leading electrode 12c extending from the coil pattern 4 are disposed on the insulating layer 6b, an insulating layer 6c is disposed on the coil pattern 4 and the leading electrode 12c, and a coil pattern 5 and a leading electrode 12d extending from the coil pattern 5 are disposed on the insulating layer 6c.
One end of the coil pattern 4 is electrically connected to the leading electrode 12a through a via hole 13a provided in the insulating layer 6b and the leading electrode 12a is electrically connected to an external electrode 11a. The other end of the coil pattern 4 is electrically connected to an external electrode 11c via the leading electrode 12c.
One end of the coil pattern 5 is electrically connected to the leading electrode 12b through a via hole 13c provided in the insulating layer 6c and a via hole 13b provided in the insulating layer 6b, and the leading electrode 12b is connected to an external electrode 11b. The other end of the coil pattern 5 is electrically connected to an external electrode 11d via the leading electrode 12d.
When the common mode choke coil 1 is assembled into a circuit by electrically connecting the individual external electrodes 11 to respective connectors of the circuit, the coil pattern 4 and the coil pattern 5 are assembled into the circuit.
Since the common mode choke coil 1 can be manufactured by a thin-film fabricating technique, for example, sputtering and evaporation, it is easily miniaturized and high productivity can be obtained.
It is important to increase the degree of electromagnetic coupling between coil patterns in a coil element such as, for example, a common mode choke coil or a transformer to improve the electrical characteristics of the coil element. For example, the above-mentioned common mode choke coil can be configured so as to have a high impedance to common mode noise, and thus, the capability of the coil element eliminating common mode noise can be enhanced. Also, a transformer can be configured so as to decrease an energy loss and to increase a bandwidth thereof.
In the common mode choke coil 1 shown in FIG. 4 and FIG. 5, since the insulating layers 6 can be made by a thin-film fabricating technique as described above, the thicknesses of the insulating layers 6 can be reduced. That is, the space between the coil pattern 4 and the coil pattern 5 can be reduced. As the space between the coil patterns 4 and 5 becomes narrower, the degree of electromagnetic coupling between the coil patterns 4 and 5 increases, and thus, the impedance of the common mode choke coil 1 can be increased.
However, in order to reliably provide a certain minimum required insulation quality between the coil pattern 4 and the coil pattern 5, the thicknesses of the insulating layers 6 cannot be reduced limitlessly. Therefore, in the method for increasing the degree of electromagnetic coupling by reducing the thicknesses of the insulating layers 6, and increasing the impedance of the common mode choke coil 1, there are limits to an amount of improvement to the electromagnetic coupling and impedance characteristics. As a result, satisfactory elimination of common mode noise cannot be achieved.